ICE: mir build: scope: `index out of bounds: the len is 0 but the index is 0`

[matthiaskrgr]

auto-reduced (treereduce-rust):

pub fn std() -> _ {
    return const {
        ({
            return ();
        })
    };
}

original:

#[non_exhaustive]
// run-pass

pub fn std() ->  _  { return  const {({ return  (
); }
)}; }

Version information

rustc 1.80.0-nightly (7717a306b 2024-05-28)
binary: rustc
commit-hash: 7717a306b2678ba9ece19b723c76a6b3a89ba931
commit-date: 2024-05-28
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.6

Command:
/home/matthias/.rustup/toolchains/master/bin/rustc

Program output

warning: unnecessary parentheses around block return value
 --> /tmp/icemaker_global_tempdir.JYB2yZP8eq2z/rustc_testrunner_tmpdir_reporting.ZJFueSdJhgaC/mvce.rs:3:9
  |
3 |         ({
  |         ^
4 |             return ();
5 |         })
  |          ^
  |
  = note: `#[warn(unused_parens)]` on by default
help: remove these parentheses
  |
3 ~         {
4 |             return ();
5 ~         }
  |

error[E0601]: `main` function not found in crate `mvce`
 --> /tmp/icemaker_global_tempdir.JYB2yZP8eq2z/rustc_testrunner_tmpdir_reporting.ZJFueSdJhgaC/mvce.rs:7:2
  |
7 | }
  |  ^ consider adding a `main` function to `/tmp/icemaker_global_tempdir.JYB2yZP8eq2z/rustc_testrunner_tmpdir_reporting.ZJFueSdJhgaC/mvce.rs`

warning: unreachable expression
 --> /tmp/icemaker_global_tempdir.JYB2yZP8eq2z/rustc_testrunner_tmpdir_reporting.ZJFueSdJhgaC/mvce.rs:2:12
  |
2 |       return const {
  |  ____________^
3 | |         ({
4 | |             return ();
  | |             --------- any code following this expression is unreachable
5 | |         })
6 | |     };
  | |_____^ unreachable expression
  |
  = note: `#[warn(unreachable_code)]` on by default

error[E0121]: the placeholder `_` is not allowed within types on item signatures for return types
 --> /tmp/icemaker_global_tempdir.JYB2yZP8eq2z/rustc_testrunner_tmpdir_reporting.ZJFueSdJhgaC/mvce.rs:1:17
  |
1 | pub fn std() -> _ {
  |                 ^
  |                 |
  |                 not allowed in type signatures
  |                 help: replace with the correct return type: `()`

thread 'rustc' panicked at compiler/rustc_mir_build/src/build/scope.rs:639:58:
index out of bounds: the len is 0 but the index is 0
stack backtrace:
   0:     0x7e0919f7bae5 - std::backtrace_rs::backtrace::libunwind::trace::h8e3ec5065424d28b
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/../../backtrace/src/backtrace/libunwind.rs:116:5
   1:     0x7e0919f7bae5 - std::backtrace_rs::backtrace::trace_unsynchronized::h882cf49e59475dbc
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/../../backtrace/src/backtrace/mod.rs:66:5
   2:     0x7e0919f7bae5 - std::sys_common::backtrace::_print_fmt::h2c337a2c4b83dc01
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/sys_common/backtrace.rs:68:5
   3:     0x7e0919f7bae5 - <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt::h5fad958b7140a660
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/sys_common/backtrace.rs:44:22
   4:     0x7e0919fcc5db - core::fmt::rt::Argument::fmt::h9f1efa968e340ff7
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/core/src/fmt/rt.rs:165:63
   5:     0x7e0919fcc5db - core::fmt::write::hc4341e028d4aa04e
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/core/src/fmt/mod.rs:1168:21
   6:     0x7e0919f7086f - std::io::Write::write_fmt::hf3dd4197197b5730
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/io/mod.rs:1835:15
   7:     0x7e0919f7b8be - std::sys_common::backtrace::_print::hee589e8f95e333af
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/sys_common/backtrace.rs:47:5
   8:     0x7e0919f7b8be - std::sys_common::backtrace::print::hc6a85e053c1c7334
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/sys_common/backtrace.rs:34:9
   9:     0x7e0919f7e2e9 - std::panicking::default_hook::{{closure}}::he62a1676f96fee54
  10:     0x7e0919f7e08a - std::panicking::default_hook::h2ceb904f12dea8be
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/panicking.rs:298:9
  11:     0x7e091678e6a0 - std[d6c1f98fb4e95ff7]::panicking::update_hook::<alloc[78b6f7e6bd40c475]::boxed::Box<rustc_driver_impl[c511ea10b869fcf7]::install_ice_hook::{closure#0}>>::{closure#0}
  12:     0x7e0919f7ea1b - <alloc::boxed::Box<F,A> as core::ops::function::Fn<Args>>::call::hc5b72f85a4688529
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/alloc/src/boxed.rs:2077:9
  13:     0x7e0919f7ea1b - std::panicking::rust_panic_with_hook::h0a690364d46daaac
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/panicking.rs:799:13
  14:     0x7e0919f7e794 - std::panicking::begin_panic_handler::{{closure}}::h9c10c03249dd0030
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/panicking.rs:664:13
  15:     0x7e0919f7bfa9 - std::sys_common::backtrace::__rust_end_short_backtrace::h599802efea5774a9
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/sys_common/backtrace.rs:171:18
  16:     0x7e0919f7e4c7 - rust_begin_unwind
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/panicking.rs:652:5
  17:     0x7e0919fc8b73 - core::panicking::panic_fmt::h16f8f3f4aae78ff5
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/core/src/panicking.rs:72:14
  18:     0x7e0919fc8d87 - core::panicking::panic_bounds_check::h601b2b36c2e3745d
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/core/src/panicking.rs:274:5
  19:     0x7e0917e79f75 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::break_scope
  20:     0x7e0917e8c214 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::stmt_expr
  21:     0x7e0917e8c1c1 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::stmt_expr
  22:     0x7e09186f5a41 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::ast_block_stmts
  23:     0x7e09186f507b - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::ast_block
  24:     0x7e0917e9e558 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::expr_into_dest
  25:     0x7e0917e961ea - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::as_temp::{closure#0}
  26:     0x7e0917e9f6fb - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::expr_into_dest
  27:     0x7e0917e9f972 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::expr_into_dest
  28:     0x7e09186f615c - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::ast_block_stmts
  29:     0x7e09186f507b - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::ast_block
  30:     0x7e0917e9e558 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::expr_into_dest
  31:     0x7e0917e9f972 - <rustc_mir_build[9415bcb3f6b13a09]::build::Builder>::expr_into_dest
  32:     0x7e0917e7481e - rustc_mir_build[9415bcb3f6b13a09]::build::mir_build
  33:     0x7e0917e712ce - rustc_query_impl[a43b8a3fae63a645]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a43b8a3fae63a645]::query_impl::mir_built::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b8101328ceeb956a]::query::erase::Erased<[u8; 8usize]>>
  34:     0x7e0917dc212e - rustc_query_system[ef4de90281b6d444]::query::plumbing::try_execute_query::<rustc_query_impl[a43b8a3fae63a645]::DynamicConfig<rustc_query_system[ef4de90281b6d444]::query::caches::VecCache<rustc_span[8ae10af61471f323]::def_id::LocalDefId, rustc_middle[b8101328ceeb956a]::query::erase::Erased<[u8; 8usize]>>, false, false, false>, rustc_query_impl[a43b8a3fae63a645]::plumbing::QueryCtxt, false>
  35:     0x7e0917dc1c4d - rustc_query_impl[a43b8a3fae63a645]::query_impl::mir_built::get_query_non_incr::__rust_end_short_backtrace
  36:     0x7e0917e657b0 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor>::visit_inner_body
  37:     0x7e09150a0d44 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_expr
  38:     0x7e09150a0fd3 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_expr
  39:     0x7e09150a0d44 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_expr
  40:     0x7e09183ea835 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_block
  41:     0x7e09150a2054 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_expr
  42:     0x7e09150a0fd3 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_expr
  43:     0x7e09150a0d44 - <rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::UnsafetyVisitor as rustc_middle[b8101328ceeb956a]::thir::visit::Visitor>::visit_expr
  44:     0x7e0917e62a24 - rustc_mir_build[9415bcb3f6b13a09]::check_unsafety::check_unsafety
  45:     0x7e0917e624c7 - rustc_query_impl[a43b8a3fae63a645]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a43b8a3fae63a645]::query_impl::check_unsafety::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b8101328ceeb956a]::query::erase::Erased<[u8; 0usize]>>
  46:     0x7e0917e615d4 - rustc_query_system[ef4de90281b6d444]::query::plumbing::try_execute_query::<rustc_query_impl[a43b8a3fae63a645]::DynamicConfig<rustc_query_system[ef4de90281b6d444]::query::caches::VecCache<rustc_span[8ae10af61471f323]::def_id::LocalDefId, rustc_middle[b8101328ceeb956a]::query::erase::Erased<[u8; 0usize]>>, false, false, false>, rustc_query_impl[a43b8a3fae63a645]::plumbing::QueryCtxt, false>
  47:     0x7e0917e61207 - rustc_query_impl[a43b8a3fae63a645]::query_impl::check_unsafety::get_query_non_incr::__rust_end_short_backtrace
  48:     0x7e0918568e5b - rustc_interface[3320d9bec8b3433a]::passes::analysis
  49:     0x7e0918567f5b - rustc_query_impl[a43b8a3fae63a645]::plumbing::__rust_begin_short_backtrace::<rustc_query_impl[a43b8a3fae63a645]::query_impl::analysis::dynamic_query::{closure#2}::{closure#0}, rustc_middle[b8101328ceeb956a]::query::erase::Erased<[u8; 1usize]>>
  50:     0x7e091899c225 - rustc_query_system[ef4de90281b6d444]::query::plumbing::try_execute_query::<rustc_query_impl[a43b8a3fae63a645]::DynamicConfig<rustc_query_system[ef4de90281b6d444]::query::caches::SingleCache<rustc_middle[b8101328ceeb956a]::query::erase::Erased<[u8; 1usize]>>, false, false, false>, rustc_query_impl[a43b8a3fae63a645]::plumbing::QueryCtxt, false>
  51:     0x7e091899bf8f - rustc_query_impl[a43b8a3fae63a645]::query_impl::analysis::get_query_non_incr::__rust_end_short_backtrace
  52:     0x7e09187f9f5c - rustc_interface[3320d9bec8b3433a]::interface::run_compiler::<core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>, rustc_driver_impl[c511ea10b869fcf7]::run_compiler::{closure#0}>::{closure#1}
  53:     0x7e09187c0609 - std[d6c1f98fb4e95ff7]::sys_common::backtrace::__rust_begin_short_backtrace::<rustc_interface[3320d9bec8b3433a]::util::run_in_thread_with_globals<rustc_interface[3320d9bec8b3433a]::util::run_in_thread_pool_with_globals<rustc_interface[3320d9bec8b3433a]::interface::run_compiler<core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>, rustc_driver_impl[c511ea10b869fcf7]::run_compiler::{closure#0}>::{closure#1}, core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>>::{closure#0}, core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>>
  54:     0x7e09187c03b2 - <<std[d6c1f98fb4e95ff7]::thread::Builder>::spawn_unchecked_<rustc_interface[3320d9bec8b3433a]::util::run_in_thread_with_globals<rustc_interface[3320d9bec8b3433a]::util::run_in_thread_pool_with_globals<rustc_interface[3320d9bec8b3433a]::interface::run_compiler<core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>, rustc_driver_impl[c511ea10b869fcf7]::run_compiler::{closure#0}>::{closure#1}, core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>>::{closure#0}, core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>>::{closure#0}::{closure#0}, core[5e8023d66ded8339]::result::Result<(), rustc_span[8ae10af61471f323]::ErrorGuaranteed>>::{closure#2} as core[5e8023d66ded8339]::ops::function::FnOnce<()>>::call_once::{shim:vtable#0}
  55:     0x7e0919f889eb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::ha3f12f701ab217ba
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/alloc/src/boxed.rs:2063:9
  56:     0x7e0919f889eb - <alloc::boxed::Box<F,A> as core::ops::function::FnOnce<Args>>::call_once::hbabc732018734ba3
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/alloc/src/boxed.rs:2063:9
  57:     0x7e0919f889eb - std::sys::pal::unix::thread::Thread::new::thread_start::h25b845304e0b166b
                               at /rustc/7717a306b2678ba9ece19b723c76a6b3a89ba931/library/std/src/sys/pal/unix/thread.rs:108:17
  58:     0x7e09134aa1cf - <unknown>
  59:     0x7e091352b6ec - <unknown>
  60:                0x0 - <unknown>

error: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/issues/new?labels=C-bug%2C+I-ICE%2C+T-compiler&template=ice.md

note: please make sure that you have updated to the latest nightly

note: rustc 1.80.0-nightly (7717a306b 2024-05-28) running on x86_64-unknown-linux-gnu

query stack during panic:
#0 [mir_built] building MIR for `std::{constant#0}`
#1 [check_unsafety] unsafety-checking `std`
end of query stack
error: aborting due to 2 previous errors; 2 warnings emitted

Some errors have detailed explanations: E0121, E0601.
For more information about an error, try `rustc --explain E0121`.

[theemathas]

Minimized:

fn foo() {
    const { return }
}

Variations that also cause ICEs of varying kinds:

fn foo() {
    'a: {
        const { break 'a }
    }
}

fn foo() {
    loop {
        const { break }
    }
}

fn foo() {
    'a: {
        const { continue 'a }
    }
}

fn foo() {
    loop {
        const { continue }
    }
}

async fn foo() {
    const { async {}.await }
}

fn foo() {
    let x = 1;
    const { &x };
}

[Noratrieb]

cc @nbdd0121 that's pretty bad... the fix for that should probably be backported or maybe even delaying the stabilization for one release? no strong opinion.

[nbdd0121]

These examples all generate correct errors on beta (which contains inline const stablisation) but ICE on nightly. Looks like something else breaks these?

[nbdd0121]

@rustbot label: +E-needs-bisection

[apiraino]

I've tried bisecting a couple of them and they all both point to 7717a30 cc @oli-obk @nnethercote

@rustbot label -E-needs-bisection

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

Being easy to be triggered even if for now only on nightly.

@rustbot label -I-prioritize +P-high